Hepatitis C virus (HCV) infection is most often clinically inapparent and rarely associated with symptoms of acute hepatitis. Most patients, however, fail to resolve the acute infection and proceed to develop chronic hepatitis with the risk of liver cirrhosis and hepatocellular carcinoma later in life. Research thus far has been hampered by the fact that patients with well documented episodes of acute, self-limited hepatitis C are rare, that the intrahepatic immune responses at the site of inflammation cannot not be prospectively studied, and that sequences of the infecting virus are often unknown and rather heterogeneous. Transfection of a chimpanzee with RNA transcribed from a full length HCV cDNA clone has now provided the opportunity to prospectively study the peripheral blood as well as intrahepatic T cell response with regards to its vigor, kinetics and specificity against antigens of a single, defined sequence. During the last year we have prospectively studied a chimpanzee, that had been intrahepatically transfected with HCV-RNAs encoding predicted lethal mutations in different genome regions. While the chimpanzee remained HCV-RNA and anti-HCV negative with normal ALTs for 32 weeks, an HCV NS3 specific T cell response was detectable in the peripheral blood even in the absence of detectable circulating virus. This finding suggests that priming of HCV specific T cell responses may occur in the absence of detectable systemic infection.After challenge with wildtype HCV RNA transcripts, the chimpanzee developed an acute hepatitis C and the specificity of the peripheral blood response was broadly directed to E1E2, NS4, NS5A and NS5B. The ALT concentrations peaked at week 10. A second, maximal proliferative T cell response was observed 6 weeks after clearance of HCV-RNA from the peripheral blood and persisted at a lower intensity during recovery and at least one year of follow-up. Remarkably, the immune response was targeted against the same HCV- helicase epitope, previously described in patients with acute, self- limited hepatitis C. The number of intrahepatic T cell clones derived from biweekly biopsies was also maximal at week 10 coinciding with the peak in both ALT and peripheral blood T cell response. While the peripheral blood T helper response was multispecific, the intrahepatic T cell response was oligospecific and focused on NS3-helicase and to a lesser extent on NS4.Our work provides evidence that 1) the priming of HCV-specific T cell responses may occur in the absence of detectable systemic infection and 2) the HCV helicase-specific T cell response, previously observed in the blood of patients with self-limited hepatitis C, may also contribute to liver injury and/or viral clearance in infected chimpanzees. Currently, we have initiated studies to determine whether this immune response is capable of protecting from reinfection by challenging the chimpanzee with increasing titers of homologous virus. The immune response and outcome of infection will be compared to a na?ve chimpanzee challenged with increasing titers of the same homologous virus. In addition, we are trying to induce a cellular immune response of comparable strength and specificity in a chronically infected chimpanzee and to analyze its effects on viral load and disease activity. Results will be relevant for our understanding of virus-host kinetics and outcome of infection as well as for vaccine development. - HCV/CTL/Immune Response/Chimpanzee